Pavement markers have become widely accepted as permanent installations for providing visible signals which mark traffic lanes and control the flow of traffic on roadways in combination with, or in place of, conventional painted traffic lines. A large number of such markers employ retroreflectors which retroreflect light emanating from oncoming vehicles to provide a signal visible to the operators of such oncoming vehicles.
Exemplary of other pavement markers previously known are those disclosed in U.S. Pat. No. 3,790,293 issued to S. A. Heenan et al., on Feb. 5, 1974, and U.S. Pat. No. 3,809,487, issued to R. M. Flanagan on May 7, 1974, both commonly assigned herewith.
With the advent of the pavement marker disclosed in Heenan U.S. Pat. No. 3,332,327, and since its introduction in 1963, millions of such pavement markers as generally disclosed in that patent have been manufactured and utilized in the "sun belt" states. Subsequent to the development of the '327 patent, various attempts were made to provide a more abrasion-resistant front surface by coating or other protective element, in order to enhance the longevity of the retroreflector element and to make it useful under more rigorous environmental conditions. Attempts have been made to coat the front surface by chemical means (such as disclosed in U.S. Pat. No. 4,753,548).
Applicants' assignee successfully developed a means for applying a micro-thin layer of untempered glass pursuant to the patented process and structure disclosed in U.S. Pat. Nos. 4,232,979, 4,340,319, and 4,596,622, all commonly assigned.
Efforts also were made over an extended period to utilize the basic invention of the U.S. Pat. No. 3,332,327 in "snow belt" states. To that end, after a long succession of efforts, applicants' assignee received, inter alia, U.S. Pat. Nos. 4,195,945 and 4,174,184 covering commercially acceptable devices. The '945 patent issued to S. A. Heenan on Apr. 1, 1980.
Other developments in the retroreflective pavement marker art have included multicellular type structures, in which a plurality of cube-corner type retroreflector elements were isolated by a series of walls around the reflective elements defining "cells". This was done primarily to preserve structural integrity and reflectivity in the event some of the cells were damaged (such as shown in 3M's Holmen U.S. Pat. No. 3,924,929; and also in U.S. Pat. Nos. 4,208,090; 4,227,772; and 4,232,979, the latter all assigned to applicant's assignee).
In the basic structure shown in the '327 patent, the plastic retroreflector elements are first formed as part of the walls of a hollow shell, and then a layer of metal, by vacuum metallization, is deposited on the exposed faces of the cube corner retroreflector elements. Following that step, the "shell" is filled or "potted" with a rigid epoxy-type material. The resulting structure is relatively rigid and over the years has proven to be remarkably durable in use. However, exposure of the metallization to moisture may lead to failure of the retroreflector. Further, because of the diminution in specific intensity of the reflected light (caused by the metallization of the retroreflector elements and distortion during the epoxy filling process, see, for example, Suhr et al U.S. Pat. No. 4,070,095 and Attar U.S. Pat. No. 4,726,706), various attempts have been made over the years to provide what has been known as an "air gap" marker, such as devices disclosed in U.S. Pat. Nos. 4,208,090; 4,227,772; and 4,232,979. While useful as temporary markers, for example during construction, none has been successfully used as a pavement marker installed for an extended time period. Among the reasons are: lack of strength of the housing; poor adhesion of the "hollow" waffle bottom housing to the roadway--when bitumen adhesive is used; and damage to the reflector cell elements.
It also is increasingly a problem in today's economy and under busy traffic conditions to shut down a road for any length of time for installation and for subsequent removal and replacement of pavement markers. Not only is it labor intensive and expensive to do so, but there is a risk of increased accidents when roads are closed or lanes diminished during construction or maintenance. It therefore is highly desirable to have a pavement marker which will have an anticipated life which is longer than that previously expected for pavement markers of the type manufactured in accordance with the U.S. Pat. No. 3,332,327, and which will produce higher initial reflectance.
One advantage of the full epoxy-filled markers is that they provide a textured and flat bottom surface, which makes them readily adherent through either bitumen or epoxy adhesive to any roadway surface. However, it has been observed that when hollow ribbed type markers are used, such as for example the type of temporary roadway marker illustrated in Luckinbill U.S. Pat. No. D-267,983, then under certain roadway conditions, particularly on an asphalt roadway and using bitumen adhesive, they are more easily dislodged than an epoxy-filled marker. That is because those hollow "waffle" type ribbed bases tend to act as a "cookie cutter" against the bitumen.
Accordingly, another advantage of the present invention is the ability to employ, in a pavement marker having a thermoplastic hollow base member, means for achieving higher retroreflectivity, by eliminating the metallizing of and potting against reflective elements, while also providing a relatively flat bottom surface to enhance the bond between the installed marker and the underlying roadway surface.
It is a primary object of the present invention to provide a pavement marker having an improved structure for optimum strength and resistance to applied forces as well as providing for an improved retroreflective system enabling the marker to extend its useful life. This is in part accomplished by providing markers on which the initial retroreflective intensity is consistently higher than that achieved from the metalized type marker and in which multiple cells segregating groups of retroreflector elements are also employed so as to prevent damage to any particular cell from propagating into adjacent areas, which will thereby increase the life of the marker as there will continue to be some number of retroreflective elements operational even though some have been damaged and thus the marker will still be usable for extended time periods.
Additionally, it is a primary object of the present invention to provide an improved pavement marker formed of particular materials which will enable it to be used on various pavement surfaces. It has been recognized that, depending upon the nature of the roadway service (asphalt vs. concrete), and upon the nature of the adhesive used to apply a raised pavement marker (bitumen vs. epoxy), the adherence of the marker to the roadway surface may vary dramatically, and so, too, its ability to withstand impact forces without fracturing the marker and/or damaging the underlying road surface.
Therefore, another object of the present invention is to provide a marker having a particular configuration for its base which will enable the marker to be inexpensively and efficiently produced while at the same time providing a new and novel combination of various elements so as to achieve the foregoing objectives.
Yet another object of the present invention is to provide an improved pavement marker which, through the use of certain selected materials, provides enhanced durability. In particular, an important feature of one embodiment of the present invention is that the reflector housing is constructed of a long-fiber-reinforced thermoplastic material having properties which demonstrate superior performance in a variety of environmental conditions from high to low temperatures. The material so used provides improved impact and compression resistance of the reflector assembly, together with excellent abrasion and mar resistance.
Still another object of the present invention is to provide an improved reflector housing having hollow recesses formed by a ribbed structure and filled with epoxy to form a flat bottom surface that is textured for better adherence to a bituminous adhesive and which will have reduced delamination and brittleness failures.
It is also an object of the present invention to provide an improved reflector housing having a flat bottom that extends substantially beyond the periphery of the housing to provide an enlarged base for greater adhesion to the adhesive attaching the housing to the road surface. These advantages of the present invention will be recognized when considered in conjunction with the following detailed description of the drawings and accompanying claims.